This invention relates to plasma display panels and pasmal-addressed electro-optic display panels, such as plasma-addressed liquid crystal (PALC) display panels.
In a plasma display device, the electro-optic medium is a plasma-producing gas. That is, such a display device utilizes the visible radiation emitted from the glow discharge of a plasma to form the display, while a PALC display device utilizes the plasma as a switch to apply data voltages to pixels of a separate electro-optic medium, generally a liquid crystal (LC) material. Visible radiation provided by backlighting is modulated by the pixels in accordance with the data voltages to form the display. In both types of devices, the glow discharge or ignition is achieved by the application of a voltage applied across cathode and anode electrodes in a plasma chamber. Herein, the term "plasma-containing display device" is used to refer generically to both plasma and PALC display devices.
PALC display devices comprise, typically, a sandwich of: a first substrate having deposited on it parallel transparent column electrodes, commonly referred to as "ITO" columns or electrodes since indium-tin oxides are typically used, on which is deposited a color filter layer; a second substrate comprising parallel sealed plasma channels corresponding to rows of the display crossing all of the ITO columns each of which is filled with a low pressure ionizable gas, such as helium, and containing spaced cathode and anode electrodes along the channel for ionizing the gas to create a plasma, which channels are closed off by a thin transparent insulator; and an electro-optic material, such as a liquid crystal (LC) material, located between the substrates. The structure behaves like an ac:ive matrix liquid crystal display in which the thin film transistor switches at each pixel are replaced by a plasma channel acting as a row switch and capable of selectively addressing a row of LC pixel elements. In operation, successive lines of data signals representing an image to be displayed are sampled at column positions and the sampled data voltages are respectively applied to the ITO columns. All but one of the row plasma channels are in the de-ionized or non-conducting state. The plasma of the one ionized selected channel is conducting and, in effect, establishes a reference potential on the adjacent side of a row of pixels of the LC layer, causing each LC pixel to charge up to the applied column potential of the data signal. The ionized channel is turned off, isolating the LC pixel charge and storing the data voltage for a frame period. When the next row of data appears on the ITO columns, only the succeeding plasma channel row is ionized to store the data voltages in the succeeding row of LC pixels, and so on. As is well known, the attenuation of each LC pixel to backlight or incident light is a function of the stored voltage across the pixel. A more detailed description is unnecessary because the construction, fabrication, and operation of such PALC devices have been described in detail in the following publication, the contents of which are hereby incorporated by reference: Buzak et al., "A 16-Inch Full Color Plasma Addressed Liquid Crystal Display", Digest of Tech. Papers, 1993 SID Int. Symp., Soc. for Info. Displ. pp. 883-886.
One of the main difficulties encountered in such displays is cathode sputtering due to heavy ion bombardment during activation of the plasma. Such sputtering leads to deposition of the cathode electrode material on the inside walls of the display, thus reducing transmission and efficacy of the display.
In addition, the ideal plasma channel would allow a short plasma formation time at low voltages; a stable data setup and capture time; and a short plasma decay time.
The present state of the art uses Cr/Cu/Cr electrodes coated with a layer of LaB.sub.6 or GdB.sub.6, and a gas fill of pure helium in the plasma channels. With this arrangement, the plasma can be switched on within 3 .mu.s by applying 350 V between the anode and the cathode electrodes within the plasma channel. While such switching time is acceptable, the plasma remains in a conductive state much longer (18 .mu.s) than is required. This results in degradation of the signal on the LC pixel and does not allow time for the use of crosstalk reduction techniques. This can be improved by using gas mixtures that have more suitable decay times such as He-Ne. However, the electrode sputtering during the plasma state worsens with this and other gas mixtures and the lifetime of the display panel degrades. Moreover, the ignition voltage required (350 V) leads to significant sputtering of electrode material.